Download notes 5.3

Chapter Five: Forces
5.1 Forces
5.2 Friction
5.3 Forces and Equilibrium
Chapter 5.3 Learning Goals
Determine the net force acting
on an object.
Define equilibrium.
Draw free-body diagrams to
represent all forces acting on a
body.
5.3 Adding forces
 To figure out if or how an object
will move, we look at ALL of the
forces acting on it.
 Four forces act on a plane:
1.
2.
3.
4.
weight
drag (air friction)
the thrust of the engines, and
the lift force caused by the flow of air
over the wings.
5.3 Forces and Equilibrium
 The sum of all the forces on an object is
called the net force.
 The word net means total but also
means the direction of the forces has
been taken into account.
In what direction
will this plane go?
5.3 Equilibrium
When several forces
act on the same
object:
1. The net force is
zero, or
2. The net force is
NOT zero.
5.3 Equilibrium
When net force is
non- zero, the
object will
accelerate in the
direction of the
net force.
5.3 Equilibrium
 When the forces
are balanced, the
net force is zero.
 When the net force
on an object is
zero, we say the
object is in
equilibrium.
 An object in
equilibrium has
zero acceleration
5.3 Equilibrium and normal forces
 A normal force is
created whenever an
object is in contact
with a surface.
 The normal force has
equal strength to the
force pressing the
object into the surface,
which is often the
object’s weight.
The normal force is
sometimes called
the support force.
5.3 The free body diagram
 How do you keep
track of many
forces with different
directions?
 Draw a free-body
diagram that
contains the
objects, like a book
on a table.
5.3 Solving equilibrium problems
 For an object to be in equilibrium,
all the forces acting on the object
must add up to zero.
Is this object in
equilibrium?
Solving Problems
Two chains are used to support a
small boat weighing 1,500 newtons.
One chain has a tension of 600
newtons.
What is the force exerted by the other
chain?
Solving Problems
1. Looking for:
 …tension on chain 2
2. Given
 …weightboat = 1,500N; tension1 = 600 N
 Implied: weight and tension are forces
3. Relationships:
 Net force on boat = zero
Solving Problems
4. Solution:
 Draw free body
diagram
 Upward force of chains = weight of boat
 600 N + tension2 = 1,500 N
 tension2 = 900 N